This invention relates to optical reading or writing apparatus such as an optical disc memory for reading or writing information by light, more particularly to the apparatus for maintaining perpendicularity between the information-carrying medium and the optic axis of the incident beam of light.
FIG. 5 is a cross-sectional view of an optical reading or writing apparatus of the prior art, as presented in Japanese Utility Model Application Laid-open No. 135817/1985 and Patent Application Laid-open No. 186237/1982. The apparatus comprises: a disc-shaped information-carrying medium 1 that rotates around a central axis and on which information can be written and read by means of light; a spindle motor 2 to turn the information-carrying medium 1; a turntable 3 that supports the information-carrying medium 1 and transmits the revolution of the spindle motor 2; a clamper 4 that rotates together with the information-carrying medium 1 and holds it against the turntable 3; a clamper mount 5 that supports the clamper 4 while permitting it to turn freely; a sliding base 6 free to slide in the radial direction of the information-carrying medium 1; an optical head 7, mounted on the sliding base 6, that illuminates the information-carrying medium 1 with a light beam; a shaft 8 that supports the optical head 7 on the sliding base 6 and enables it to be tilted; an objective lens 9 mounted on the optical head 7, for directing the light beam onto the information-carrying medium 1, the lens being free to move both parallel to the optic axis 10 (in the focusing direction) and perpendicular to the optic axis 10 (in the tracking direction); and a tilt servo mechanism 11 for tilting the optical head 7 with the shaft 8 as a pivot. The optical head 7 and related components are shown in two positions in this drawing, one position near the center of the information-carrying medium 1 and one position near the circumference.
FIG. 6 is an enlarged view showing a detection means 12 for detecting any deviation from perpendicularity between the optic axis 10 and the surface of the informationcarrying medium 1. The detection means 12 comprises a lightemitting element 13, photosensors 14 and 15, and an operational amplifier 16 for the signals output by the photosensors 14 and 15. The detection means 12 can be mounted on the optical head 7 with the photosensors 14 and 15 located at equal distances from the light-emitting element 13. Any inclination of the detection means 12 with respect to the surface of the information-carrying medium 1 causes a difference in the intensity of light received by the two photosensors 14 and 15. The signal output from the operational amplifier 16 therefore indicates the inclination between this surface and the optic axis.
In an optical reading or writing apparatus with the structure of the prior art as described above, the information-carrying medium 1 is held between the turntable 3 and the clamper 4 as shown in FIG. 5 and rotated by the spindle motor 2. Due to shrinkage immediately after manufacture, aging changes, temperature variations, and other factors, the information-carrying medium 1 is generally warped into a concave shape as shown in the drawing.
When the sliding base 6, which slides in the radial direction of the information-carrying medium 1, reaches a position under the non-horizontal part of the surface near the circumference of the information-carrying medium 1, the optic axis 10 of the light beam from the optical head 7 mounted on the sliding base 6 is no longer perpendicular to the recording surface of the information-carrying medium 1. This state is detected by the detection means 12 shown in FIG. 6. In response to the signal output from the operational amplifier 16, the tilt servo mechanism 11 then operates to tilt the optical head 7 around the shaft 8 until perpendicularity between the optic axis 10 and the surface of the information-carrying medium 1 is restored.
If the optic axis 10 were permitted to remain nonperpendicular to the surface of the information-carrying medium 1, the spot of light would be focused onto the surface in a distorted shape. This causes such problems as follows. Namely, in information recording, the pits (holes) representing the information would be formed inaccurately on the surface, and in information reading, a carrier-to-noise ratio is reduced and a number of errors is increased. It would also become difficult to maintain tracking control; that is, to keep the spot right on the track on or from which the information should be recorded or reproduced. In digital optical disc systems using a diffraction method of tracking control to keep the spot right on the track, there would be considerable error in writing the information signal.
Since these problems are the result of nonperpendicularity between the optic axis 10 and the surface of the information-carrying medium 1, the tilt servo mechanism 11 tilts the optical head 7 to maintain a perpendicular relationship. The objective lens 9 also moves at the time of tilting, so the distance between the objective lens 9 and the surface of the information-carrying medium 1 undergoes considerable variation. In FIG. 5, the distance varies by as much as A to B between locations at which the surface of the information-carrying medium 1 is horizontal and locations at which it is not. In compensation for this variation, the objective lens 9 is moved parallel to the optic axis 10 to keep the light beam in focus on the recording surface of the information-carrying medium 1.
A problem in the optical reading or writing apparatus of the prior art as described above is that since the objective lens 9 must be sufficiently movable parallel to the optic axis 10 to adjust the focus, and the objective lens 9 is mounted on the optical head 7, the optical head 7 has to be fairly large. A large tilt servo mechanism 11 is also required to tilt the optical head 7. The large mass that must therefore be driven on the sliding base 6 in the radial direction of the information-carrying medium 1 raises an obstacle to high-speed driving (high-speed access to the information).